Question

Which element has the smallest atomic radius (here)?
A. $\text{Na}$
B. $\text{Mg}$
C. $\text{B}$
D. $\text{N}$

Answer 1

The options contain the elements from the different periods and different groups. Atomic radius is actually the distance between the nucleus and the last electron in that element. Check the general trends of atomic radius while moving along the period and group.

Complete answer:
Let us see the factors on which the atomic radius and the general trends of atomic radius:

Factors on which atomic radius depends	Definitions	Factor increases with	See the general trend	Effect on atomic radius
Electron shells	The circular orbitals around the nucleus in which electrons are revolving or energy shells	The principal quantum number $\left( \text{n} \right)$.	Increases atomic radius	Increases while moving down the group
Nuclear charge	The electrons and protons are oppositely charged, so there is an attractive force between the two.	Atomic number	Decreases atomic radius	Decreases along period
Shielding by electrons	When outer electrons feel repulsive force from the already present inner electrons	Number of inner electrons from that electron	Increases atomic radius	This reduces the effect of nuclear charge

A. $\text{Na}$: It is sodium which belongs to the third period and first group. According to the general trend, sodium will have maximum atomic size in its period and size decreases along the period and sodium is the first element.
B. $\text{Mg}$: It is magnesium which belongs to the third period and second group. Magnesium is placed just after sodium. According to the general trend, magnesium will have the second highest atomic size in its period after sodium because size decreases along the period and sodium is the first element.
C. $\text{B}$ : It is boron which belongs to the second period and thirteenth group. According to the general trend, boron has smaller size than metals and greater than non-metals. Atomic size decreases along the period and lithium is the first element.
D. $\text{N}$: It is nitrogen which belongs to the second period and fifteenth group. According to the general trend, nitrogen will have less atomic size than boron in its period.
The order of atomic size is $\text{Na}>\text{Mg}>\text{B}>\text{N}$. Nitrogen has the smallest atomic size.

The correct option is option ‘d’.

Note:
The size of inert gases does not follow the general trend of the periodic table. The reason is inert gases have Van der Waal radius. Inert gases do not form covalent bonds. The Van der Waals radius is larger than the covalent radius. Thus, the inert gases have the largest atomic radius in their respective periods.